1. Field of the Invention
The present invention relates to a reset circuit. More particularly, the present invention relates to an improvement of a reset circuit to be utilized in an electronic apparatus using a microcomputer.
2. Description of the Prior Art
FIG. 1 is a schematic block diagram showing structure of a conventional reset circuit; FIG. 2 is a diagram showing a specific example of the conventional reset circuit shown in FIG. 1; and FIG. 3 is a waveform diagram showing operation of the FIG. 2 reset circuit at the time of turning on of the power source.
In an electronic apparatus containing a microcomputer, for example, a reset circuit as shown in FIG. 1 is employed for the purpose of resetting an internal logical circuit and the like. When in such a reset circuit, an input signal is supplied to a reset internal circuit 2 through an input terminal 1, a reset signal is generated from the reset internal circuit 2 so as to be supplied to an internal circuit 3. The internal circuit 3 is reset by this reset signal.
The reset internal circuit 2 comprises as shown in FIG. 2, an inverter 21 for inverting the polarity of the input signal. An input terminal of the inverter 21 is connected to a DC power source (not shown) through a resistor 22. The input terminal 1 is connected in a reference potential, that is, a ground level for example, through a capacitor 4. In an ordinary microcomputer, the resistor 22 and the inverter 21 are contained in an integrated circuit, while the capacitor 4 is attached externally. Consequently, the reset internal circuit 2 shown in FIG. 1 contains the resistor 22 and the inverter 21.
The reset circuit shown in FIG. 2 performs reset operation at the rise of DC voltage at the time of turning on of the power source. In the following, the reset operation will be described, with reference to FIG. 3. FIG. 3 shows waveforms concerning DC voltage a at the time of turning on of the power source, output voltage b of the inverter 21, voltage c in the reset input terminal 1 and transition voltage V.sub.TR1 of the inverter 21, the abscissa representing time t and the ordinate representing voltage V.
First, at the time t.sub.0, when the power source turns on, the DC voltage a rises. At this time, since the resistor 22 and the capacitor 4 constitute an RC series circuit, the voltage c of the reset input terminal 1 increases with a time constant determined by the resistor 22 and the capacitor 4. The transition voltage V.sub.TR1 of the inverter 21 rises in proportion to the rise of the DC voltage a.
The inverter 21 provides an "H" level when the input applied thereto is lower than the transition voltage V.sub.TR1, and it provides an "L" level when the input applied thereto is higher than the transition voltage V.sub.TR1. Accordingly, in a period from t.sub.0 to t.sub.1 when the input voltage of the inverter 21, namely, the voltage c of the reset input terminal 1 is smaller than the transition voltage V.sub.TR1, the output voltage b of the inverter 21 increases toward the "H" level so as to be maintained in the "H" level.
However, when the voltage c of the reset input terminal 1 becomes equal to the transition voltage V.sub.TR1 at the time t.sub.1 and then the magnitude relation therebetween is reversed, the output voltage b of the inverter 21 dcscends to the "L" level to be maintained thereafter in the "L" level. Accordingly, the output voltage b of the inverter 21 has a pulse shape and the internal circuit 3 is reset in a period of the "H" level of the pulse shape.
FIG. 4 is a schematic block diagram of a microcomputer circuit in which the conventional reset circuit shown in FIGS. 1 and 2 is applied. In this block diagram of FIG. 4, an internal circuit 3 comprises a programmable counter 31, a programmable ROM 32, an arithmetic and logic unit (ALU) 33, a RAM 34, an input-output control circuit 35, an input-output port 36 and an external transistor 37. To the programmable counter 31 and the input-output control circuit 35, a reset signal is supplied from the inverter 21. After the programmable counter 31 is reset in response to a reset signal, the programmable counter 31 counts clock pulses and supplies the output of the count to the programmable ROM 32 as an address signal. The programmable ROM 32 reads out a program from a predetermined address according to the address signal to supply the program to the arithmetic and logic unit (ALU) 33. The ALU 33 performs processing operation based on the program and stores the data in the RAM 34. On the other hand, the input-output control circuit 35 is reset in response to a reset signal to control the external transistor 37 through the input-output port 36. The collector output of the external transistor 37 serves as a control signal for turning on and off the power source of a television receiver for example.
The input-output port 36 shown in FIG. 4 serves as an input port and an output port and if this port serves as an output port at the time of turning on of the power source, an unfavorable influence might be exerted on an external system. More specifically stated, since the logical level of each circuit is not fixed till the ALU 33 executed the program of the programmable ROM 32 to set the internal state, the logical level is either the "H" level or the "L" level in case of the output state of the input-output port 36. If the input-output port 36 which is to operate as an input port is in the output state, an external signal output and an output of the microcomputer will be connected within the input-output port 36. If these outputs have different potentials, it is feared that a large current flows, causing damage to the microcomputer or the peripheral circuits.
For this reason, the input-output port 36 is controlled by the input-output control circuit 35 so that it serves as an input port in a period in which the output voltage of the inverter 21 is of the "H" level after the power source is turned on. At the time t.sub.1 shown in FIG. 3, the output voltage of the inverter 21 becomes the "L" level and in consequence the microcomputer performs an initializing routine set in the programmable ROM 32. At this time, the microcomputer sets the respective components in the initial state and sets the input-output port 36 to serve as an output port. After such operation, the microcomputer advances to the main routine to repeat the operation in the main routine so that a normal state of operation of the system is maintained.
However, in case where a microcomputer to be operated is contained on a print-circuit board and is connected with other components of the circuit, the operation of the microcomputer is liable to be influenced by external noise. If the noise is applied to the reset input terminal 1, the output voltage b of the inverter 21 is caused to be in the "H" level and the program returns to the initializing routine although it should repeat the operation in the main routine. In other words, the system malfunctions due to the external noise. Let us take an example in which the external transistor 37 controls the turning on and off of the power source of a television receiver, the power source being turned on with the "L" level of the collector of the external transistor 37. In such an example, if the input-output port 36 is made to be in the input state due to external noise, the collector of the external transistor 37 becomes in the "H" level to turn off the television receiver, which sometimes causes a serious defect such as disappearance of the picture.